This invention relates to the lubrication of metallic workpieces for forming processes and, more particularly, to a dry-film, water-base lubricant comprising finely divided molybdenum disulfide (MoS.sub.2) powder; a workpiece coated with this lubricant; and the process for coating workpieces with this lubricant.
In forming operations such as heading, extrusion, or wire drawing, the workpiece being formed is forced into face-to-face contact with a die under very high pressure and pressed or drawn into a desired shape and size, during which the contacted surfaces of the workpiece and the die move laterally with respect to each other. Although some of the more ductile metals can be successfully formed without a lubricant, most metals to be headed, drawn, or extruded are lubricated to prevent galling of the work metal or the dies, sticking in the dies, and excessive die wear.
The lubrication of hard-to-work metals in cold heading processes has been a particularly difficult problem. Cold heading is a cold (less than about 350.degree. F or 177.degree. C) forming process in which the force developed by one or more strokes of a heading tool is employed to upset, or displace, the metal in a portion of a wire or rod blank to form a section of different contour or, more commonly, of larger cross section than the original. Heretofore, a hard-to-work metal, such as for example stainless steel, which is to be cold headed has been usually electroplated with a soft metal, such as copper, which acts as a cold heading lubricant, often with an additional coating of an oil or MoS.sub.2 powder lubricant. After fabrication, the copper coating usually must be removed, most commonly by treating the workpieces in heated dilute nitric acid.
Until recently the copper-bearing nitric acid waste solution was dumped, either with or without neutralization, into landfills and streams. These disposal practices are no longer permitted in many jurisdictions because of the toxicity of the copper compounds. Therefore, disposal of the copper waste solution has become a very costly problem.
MoS.sub.2 powder is a dry lubricant of such low toxicity that it is commonly used on chains and bearing surfaces in food processing equipment such as ovens. Because of its low toxicity, the disposal of molybdenum compounds into landfills or waterways is permitted so that disposal of an MoS.sub.2 lubricant after removal from the work product can be economically carried out.
Although MoS.sub.2 powder has long been used as a lubricant, there are a number of problems in using this material on production scale forming such as heading or drawing operations. In the case of multistage forming operations, enough MoS.sub.2 must be put on the workpiece to keep it lubricated when under extremely high pressure, and that MoS.sub.2 must be tightly adhered to the workpiece. Further, the MoS.sub.2 coating must be uniformly applied so that all working surfaces are adequately protected, and so that the workpiece is uniformly sized to fit into the dies. In addition, the coating should be economically applied at production rates to feedstock, particularly to wire feedstock.
Since MoS.sub.2 powder will not adhere well by itself to a metal workpiece, it has been found desirable to coat the workpiece by mixing MoS.sub.2 powder with other materials which will form a uniform and adherent coating or film on the metal. One method of applying MoS.sub.2 powder is by mixing it with grease and oils as described in Cooper et al U.S. Pat. No. 2,156,803, May 2, 1939, or by mixing it into a water-based grease as described in Freier U.S. Pat. No. 3,249,538, May 3, 1966, or as in Brown et al. U.S. Pat. No. 3,350,307, Oct. 31, 1967. However such greases do not adhere well enough to workpieces and are quickly rubbed off in multistage forming operations.
Another method is to form a dry film coating containing the MoS.sub.2 powder on the surface of the workpiece. This can be done by applying a suspension of MoS.sub.2 powder in a carrier liquid to the metal surface and then drying off the carrier to leave a dry film which should include agents to adhere the MoS.sub.2 powder to the metal substrate. For ease of handling and economy, water has been found to be an excellent carrier liquid. Since water will not wet the surface of MoS.sub.2 particles, it is necessary to include a water soluble wetting agent in such a lubricating composition.
McBride U.S. Pat. No. 2,619,458, Nov. 15, 1952, describes a method of applying a dry lubricating film containing MoS.sub.2 powder to bearing surfaces. The method comprises mixing the MoS.sub.2 into an aqueous suspension with a "suspending" agent, a "wetting" agent, and a chromate "corrosion inhibitor". The "suspending" agents are water soluble materials among which is included methyl and ethyl cellulose. However, as will be discussed further hereinbelow, that coating does not provide the unique properties of the lubricant of the present invention and includes one of several chromate compounds in all of its formulations which are highly toxic and subject to strict disposal regulations.
Silwones U.S. Pat. No. 3,244,625, Apr. 5, 1966, relates to a dry film lubricating composition comprising MoS.sub.2 powder, water soluble silicates as "film forming compounds", and toxic "adhering agents" comprising a phosphate and either borate or chromate salts, which after application is baked for several hours. The lubricant is intended for use at elevated temperatures up to about 2000.degree. F (1093.degree. C) but leaves much to be desired for cold forming operations.